I would say that depends on your expectations. If your expectations are that by pushing a button, you will magically receive the perfect lineup for each and every night and therefore win boatloads of money, then no don't bother. If you plan on using it as a statistical tool to help guide you and make better drafting decisions, then yes optimizers can be of great benefit.
I would never recommend just blindly using picks that lineup optimizers generate day in and day out. However, I must admit some generators I have used (including the top sites mentioned above) have indeed created top 5 placing lineups in GPP tournaments. They are extremely helpful at filling out remaining spots on your roster after you have locked in your top picks of the slate. Lineup optimizers are telling you, statistically speaking, the best players to draft that day based on matchups and adjusting for salaries depending on which site you are playing at. Any professional DFS player will tell you stats are extremely important but they are not everything, your gut and own insights need to be taken into account as well. You do, however, need the help of DFS tools to fill in the advanced statistical side for you if you truly want to be a successful daily fantasy sports player.